Among electric vehicle driving devices, particularly an electric vehicle driving device which directly drives a wheel is called an in-wheel motor. The in-wheel motor mentioned herein is a driving device which is installed near a wheel provided in an electric vehicle. Furthermore, the in-wheel motor may not be necessarily accommodated inside the wheel. The in-wheel motor needs to be disposed inside the wheel or near the wheel. However, the inside of the wheel or the vicinity of the wheel is a comparatively narrow space. Accordingly, a decrease in size of the in-wheel motor is demanded.
Here, the in-wheel motor rotates a vehicle wheel by transmitting a generated driving force to the vehicle wheel. Patent Literature 1 discloses a mechanism which transmits a rotational driving force generated by a motor body to a wheel through a transmission member. The transmission member includes a driving shaft to which the driving force of the motor body is transmitted and a hub which transmits the driving force transmitted to the driving shaft to the wheel. The hub is a bearing, an outer race is connected to the driving shaft and the wheel, and an inner race is fixed to a motor casing.
As the in-wheel motor, a type with a speed reduction mechanism and a direct drive type without a speed reduction mechanism are known. The in-wheel motor of the type with the speed reduction mechanism may easily ensure a rotational force sufficient for driving the electric vehicle when starting the electric vehicle or climbing a slope (climbing a slope road). However, since the in-wheel motor of the type with the speed reduction mechanism transmits a rotational force to the wheel through the speed reduction mechanism, a friction loss occurs in the speed reduction mechanism. In the in-wheel motor with the speed reduction mechanism, the rotation speed of the output shaft of the motor is normally faster than the rotation speed of the wheel. Accordingly, in the in-wheel motor of the type with the speed reduction mechanism, particularly when the electric vehicle runs at a high speed, an energy loss increases due to a friction loss in the speed reduction mechanism.
On the other hand, since the in-wheel motor of the direct drive type transmits the rotational force to the wheel without through the speed reduction mechanism, the energy loss may be reduced. However, the in-wheel motor of the direct drive type may not amplify the rotational force by the speed reduction mechanism. Accordingly, the in-wheel motor of the direct drive type may not easily ensure a rotational force sufficient for driving the electric vehicle when the electric vehicle starts or climbs a slope. As a technique for ensuring a rotational force sufficient for driving the electric vehicle, for example, Patent Literature 2 discloses a technique which includes a speed reduction mechanism having a planetary gear mechanism and two motors instead of the in-wheel motor.